Oxy-combustion of fuels is a known strategy for increasing heat utilization (thermal efficiency) in industrial furnaces relative to air-based combustion. Oxy-fuel burners have higher flame temperatures which increase radiative heat transfer from the flame to the load. Higher flame temperatures, however, can have negative consequences in certain circumstances, especially for lower temperature furnaces such as for secondary Aluminum melting. Due to the high flame temperature, the propensity of NOx formation is increased. Therefore, if N2 enters the combustion zone either through the fuel or due to air infiltration into the furnace, NOx formation can increase significantly. Also, the higher temperature flame can cause hot spots in the furnace or adversely affect the product quality. In certain cases like melting of Aluminum, the high flame temperature can also increase the rate of metal oxidation, thus resulting in metal losses.
To overcome the above issues, distributed combustion has been developed as a strategy for performing oxy-combustion at lower but very uniform temperatures. Also called diluted combustion, mild combustion, or flameless combustion (in certain circumstances when the flame is no longer visible), the central idea of this strategy is to dilute the reactants with furnace gases (mostly mixture of H2O and CO2) before combustion so as to achieve a lower and more uniform temperature distribution within the furnace. The temperature of the diluted mixtures should be kept above auto-ignition temperature to sustain the flameless mode. Unlike complex interaction between mass transport and chemical reaction observed in conventional combustion processes, the highly diluted reactants make combustion a kinetic-limited process by increasing time scale of the combustion reaction. This slow combustion process manifests itself through highly distributed reaction zones where the peak temperature is low thereby reducing NOx drastically.
Many have proposed burners for distributed combustion.
WO2004/029511 utilizes an ejector effect produced by a burner's oxygen nozzles to provide internal recirculation of furnace gases. Downstream injection of fuel allows the oxygen to mix with the furnace gases before reaching the fuel. WO2004/029511 includes 6 oxygen supply pipes placed in a circle around the fuel injection. The oxygen supply pipes preferably deliver oxygen at supersonic velocities.
As with the WO2004/029511 burner, U.S. Pat. No. 6,007,326 concerns combustion with low concentrations of both fuel and oxygen in the furnace. Dilution of the reactants is obtained with spatially separated injections of them at high velocities. The fuel and the oxidant can be preheated to any temperature above ambient.
U.S. Published Patent Application US 20070254251 discloses a burner designed for a flameless combustion regime. It includes several fuel and oxidant injections, playing different roles. A possible central flame stabilizer is surrounded by multiple nozzles for injecting fuel and gaseous oxidant into the furnace or combustion zone. It can use air or oxygen as oxidant.
Some distributed combustion burners utilizing oxygen must rely upon high velocity injections of the reactants. The high velocity injections normally require high pressures of oxygen and natural gas for operation. Because of this drawback, there is a need to achieve distributed combustion with a burner utilizing oxygen at relatively lower pressures.
Regardless of the pressure of the oxidant supply, distributed combustion is usually achieved by separated injection of fuel and oxidant into the furnace. Either one or both reactant jets are injected into the furnace in such a way as to facilitate entrainment of furnace gases into the jets, e.g. by using high velocity gradients, swirling flows or bluff bodies. The distance between the jets is determined with the objective of achieving sufficient dilution of one or both reactants before the two reactant streams interact/mix with each other. For example, U.S. Pat. No. 5,961,312 discloses a burner design wherein the distance between the fuel and air jets, L, is given by the equation: (L/Da)×[(Va/Vo)0.5]>10, where Da is the diameter of the air nozzle, Va is the velocity of air and Vo is unit velocity of air (1 m/s). Similarly, U.S. Pat. No. 6,007,326 requires a distance of at least 6 inches and preferably 24 inches between fuel and oxidant jets to achieve diluted combustion conditions for low NOx production. These spacing requirements between jets can often make burners prohibitively large and bulky.
Sometimes, a non-zero angle of injection between the reactant nozzles is also used to delay mixing of the reactants until they are diluted by furnace gases. For example, U.S. Pat. No. 5,772,421 discloses a burner design in which the fuel and oxidant are discharged such that they initially diverge away from each other but eventually mix within the furnace. However, the mixing of the diverging jets is dependent upon furnace geometry, burner operation and the location of the burner within the furnace. As a result, these burners are often effective only in certain specific furnaces and under specific operating conditions.
Another strategy to achieve distributed combustion is to distribute one of the reactants in the furnace by using multiple nozzles. The other reactant is usually supplied as a high velocity or high swirl jet to entrain furnace gases. For example, U.S. Pat. No. 6,773,256 discloses a burner in which a small quantity of fuel is supplied into the oxidant stream to achieve a fuel-lean flame. The remaining fuel is supplied via multiple fuel nozzles at fixed distances from the flame. The fuel nozzles can be designed to inject the fuel at different angles to the flame depending on the staging desired. Such a design strategy can result in a relatively large, complex burner that can be relatively expensive to manufacture and hard to control.
Because of the above-described drawbacks, there is a need to achieve distributed combustion with a simple, compact burner.
Even if burner compactness is not considered an important factor and a burner is optimized for a particular furnace configuration and for particular operating conditions, once the distance between fuel and oxidant injections is set, it is impossible to modify this distance in conventional burners with fixed injections of fuel and oxidant. This becomes important if it is desirable to provide a single burner design capable of combusting gaseous, liquid, and solid fuels. This is because different fuels have different the optimal injection conditions. This difference among optimal injection conditions even exists between different types of a single general class of fuels, such as pulverized coal.
Because of the above-described drawbacks, there is a need to achieve distributed combustion with different types of fuels with a single burner design.
One of the important conditions for achieving highly staged combustion is high furnace temperature. In order to maintain complete combustion inside the combustion chamber for highly staged combustion, the furnace must be preheated to above 800° C. Most of the highly staged burners require a preheater burner for achieving desired furnace temperatures prior to staging. For example, WO 2006/031163 discloses a burner that can be operated in both flame and staged mode. Initially when the furnace is cold, fuel and oxidant are injected from coaxial opening (pipe-in-pipe) to have a stable flame. Once the furnace temperature reaches the auto-ignition temperature of fuel, the fuel and oxidant are injected from openings that are spatially separated from each other to have a distributed combustion inside the furnace. The issue with almost all of the staged burner designs is their often poor performance at burner powers other than nominal design power. Typically these burners operate very well at nominal power conditions, however, their combustion efficiency and emission characteristics often decline significantly the moment burner power is changed from nominal to some other power. Such a change in burner power is a very common scenario for most industrial furnaces.
Because of the above-described drawback, there is also a need for a burner that can achieve satisfactory distributed combustion at a variety of burner powers.